Gene expression analysis of bovine oocytes with high developmental competence obtained from FSH-stimulated animals.

Recent progress in the ovarian stimulation protocol used for bovine in vitro maturation and fertilization, especially through optimization of the follicle-stimulating hormone (FSH) withdrawal period ("coasting") after ovarian pre-treatment with FSH, has significantly improved blastocyst outcome. Despite this important success, the underlying factors leading to improved oocyte quality have not yet been identified. The aim of this project was to compare the transcriptome of germinal vesicle-stage oocytes collected from FSH-stimulated cows after various coasting periods (20, 44, 68, and 92 hr) to determine which transcripts were accumulated or depleted during the rise and fall of competence. Oocytes from each coasting period were compared to the three other times (optimal conditions, 44 and 68 hr; under-matured, 20 hr; and over-matured, 92 hr) per animal, allowing each cow to be its own control (24 collections). Microarray analysis revealed that between 5 and 338 transcripts were significantly different across the six comparisons, with an important longitudinal modulation in terms of gene expression profile. Not surprisingly, as the transcriptional activity decreased in these oocytes, several transcripts that are significantly modulated during coasting are related to RNA processing functions, as shown by functional analysis. Ingenuity Pathway Analysis also highlighted another important function: the control of chromosome segregation. The results presented here indicate that the quality gained with the optimal coasting time does not last, and also suggests a possible mechanism of control by transcript degradation that could be implicated if the oocyte is not ovulated at the right time.